In some tunnel pasteurisers, food products to be pasteurised, packaged in bottles, cans or other containers, are fed by a conveyor along a forward movement path, which is usually divided into three main processing zones: a heating zone in which the product temperature is gradually raised; a heat-treatment zone in which the product temperature is brought to, and kept at, a pasteurising temperature for a desired time interval; and a cooling zone in which the product temperature is gradually lowered up to a desired output temperature.
At the end of the temperature processing (i.e. at the output of the pasteuriser), it may have been necessary for the food products to have been kept above a predetermined temperature for at least a predetermined time, so as to have accumulated at least a predetermined quantity of pasteurisation units (PUs). In this way, effective pasteurisation of the processed product may be assured.
Each processing zone is, in turn, divided into a plurality of sub-zones, each one of which is operable and controllable independently from the others. The products, in each sub-zone, are processed by injecting thereon a flow of a fluid at a desired predetermined temperature. Usually, the fluid is a liquid, such as water, which is injected, sprayed, or applied in any equivalent manner on the products, although other fluids may as well be used, like a gas, such as air.
For example, each sub-zone comprises a spraying unit positioned above the forward movement path of the products, and, in a possible solution, at least one collection tank positioned below the same forward movement path to collect the liquid, sprayed by the spraying unit, after it has wet the products.
According to some methods, the processing liquid fed to the spraying units of the various sub-zones is taken, at least in part, from the collection tanks.
For example, a process of recirculation is implemented, according to which the liquid fed to the spraying units of the heating sub-zones is taken from the tanks of the cooling sub-zones, whilst the liquid fed to the spraying means of the cooling sub-zones is taken from the tanks of the heating sub-zones. In this way, it is possible to achieve an effective energy saving, thanks to the fact that heat transferred to the processing liquid by the products in the cooling zones may be used to heat the products in the heating zones, while the cooled down processing liquid collected in the heating zones may be used to cool down the products in the cooling zones. Moreover, in the heat treatment zone, the liquid fed to the spraying means of each sub-zone is usually taken directly from the collection tank of that sub-zone.
Suitable temperature-control elements are also envisaged, in order to keep the processing liquid at the desired temperatures in the various zones and sub-zones of the pasteuriser (by warming or cooling the same processing liquid), e.g. including heat exchangers and/or chillers with refrigerating liquid (such as water).
FIG. 1 shows a graph, with time on the x-axis and both temperatures and pasteurisation units on the y-axis, relating to a pasteurisation treatment in the pasteuriser discussed above.
The graph refers to a system operational condition in which each product is fed at a constant speed.
The graph shows: the product theoretical heating thermal trend (dotted curve t); the temperature of the processing liquid injected on the products (continuous curve T); and the accumulation trend of product pasteurisation units (bold continuous curve PU).
Since the speed of the conveyor is assumed constant, the graph may also be interpreted as a portrait of the system condition at each given moment. In this case, the x-axis shows the position of each product along the forward movement path, whilst the y-axis shows the values of the temperature t of the products, the temperature T of the processing liquid and the quantity of pasteurisation units PU accumulated for each product at that specific position.
Accordingly, FIG. 1 also shows the heating zone A, the heat treatment zone B and the cooling zone C. By way of example, each zone is shown as being divided in four subzones, with the direction of the product forward movement being from left to right, as shown by the arrow.
As previously explained, the various temperature-control elements along the path of the products are controllable so that the temperature of the processing liquid in each zone and sub-zone is kept at a desired level, depending on the requirements specific to the products being processed, in order to assure that the products are treated at an appropriate temperature.
For example, in many applications, in order to guarantee the final quality of the processed product, it is required that the output temperature of the products at the end of the treatment is maintained in a desired range, and a control action is required so that the output temperature is not higher than an upper output temperature threshold, nor below a lower output temperature threshold.
In some methods, control of the output temperature is achieved by regulating the temperature of the processing liquid injected on the products in the various sub-zones of the cooling zone of the pasteuriser. For example, for each sub-zone of the cooling zone, the control actions on the temperature-control elements is such that the temperature of the processing fluid is kept below a respective upper limit value.
By way of example, in the pasteurisation of beer products, the upper limit value for the temperature of the processing liquid in the first sub-zone of the cooling zone may be set to 52° C.; while the upper limit value for the temperature of the processing liquid in the last sub-zone of the cooling zone may be set to 29° C., in order to guarantee a desired range for the output temperature of the products, e.g. comprised between 28° C. and 33° C.
The Applicant has realized, however, that the above known control system may have some drawbacks.
For example, it is based on a proper choice for the upper limit values of the temperatures of the processing liquid in the various sub-zones of the cooling zone; these upper limit values are chosen based on empirical data and optimization choices made by expert personnel, and, accordingly, may be biased by errors.
Moreover, problems may arise when the tunnel pasteuriser undergoes unforeseen conveyor stops or when the moving conveyor is not loaded, or it is not fully loaded, with products. When one of these conditions occur, management of the fluid temperatures in the various processing zones becomes critical, since the thermic regenerative effect between the cooling and heating zones is reduced or absent.
Indeed, when the conveyor is stopped and the product forward movement is interrupted, a general temperature increase occurs in all the zones of the pasteuriser, which may lead to unnecessary injection of refrigerating liquid in the cooling zones (due to the exceeding of the respective upper limit temperatures). Not only this represents a waste of energy, but it may also lead to errors in the output temperature of the products, which may fall outside the desired range (sometimes being lower than the minimum required temperature, other times being higher than the maximum tolerable temperature).
A similar unnecessary power consumption may also occur when the conveyor is not loaded with products, due to the general increase of temperature associated to the reduction of the heat recirculation effect.